This invention relates to hybrid motor vehicles having several power sources including an internal combustion engine and hydraulic pressure. More particularly, the invention pertains to the control of the engine and line pressure for such vehicles.
A hydraulic hybrid powertrain consisting of a number of free piston engine-pump banks, an accumulator containing hydraulic fluid at relatively high pressure, and pump/motors located at each axle for driving the wheels.
In such a system, there is a mismatch in operating efficiency of the free piston engine-pump and the efficiency of the pump/motors. The assembly that includes a free piston engine driving a hydraulic pump has a higher efficiency at higher pressures. The pump/motors have higher efficiencies at lower pressures. There is an optimum efficiency point where the combined efficiencies of these components produce overall vehicle efficiency. The flow rate produced by the engine is directly proportional to the number of operating cylinders and the engine speed. The power output by the engine is closely related to line pressure. Therefore, to insure optimum fuel economy, a target line pressure is desired at or near this optimum system efficiency point.
In such a system, there is also control activity called “line pressure cycling,” which is characterized by repetitively rising and falling line pressure. This cycling is a result of the inability to match the power input of the engine with the power output of the driving pump/motors.
Line pressure cycling is caused by two factors. If the minimum power generated by the free piston engine is greater than that currently required by the vehicle, the number of operating engine cylinders must be changed repetitively to maintain target pressure. For example, in a typical vehicle driving cycle, the engine may be on only 25% of the time. Regenerative braking, a technique used to recover kinetic energy of the vehicle during wheel braking, also causes hydraulic line pressure cycling. The recovered kinetic energy is stored in a hydraulic fluid accumulator, connected hydraulically to line. This source of energy increases line pressure when the energy is being recovered, but use of the recovered, stored energy causes a decrease in line pressure. These two factors ensure that there will be cycling around a desired target line pressure.
Therefore, any hydraulic hybrid vehicle controls system that is optimized for fuel economy, must allow cycling, and should cause the average system pressure to approach an optimal target pressure. The control system of this invention produces this result by turning on one or more of the free piston engine-pump banks as required to meet the target line pressure.